Silver/silver chloride (Ag/AgCl) ion-sensitive electrodes comprising a layer of silver in electrochemical contact with a layer of silver chloride are well known in the art, as evidenced by the following U.S. Pat. Nos. 3,883,495; 3,502,560; 3,591,482 and 3,856,649, as well as Research Disclosure Publication No. 16113, Volume 161, dated September, 1977.
Such electrodes, in addition to their utility as reference electrodes, are useful for the determination of Cl.sup..crclbar. in aqueous solutions. However, they are also extremely sensitive to the presence of uric acid, Br.sup..crclbar., and other low molecular weight materials which affect their ability to accurately measure Cl.sup..crclbar. activity and, consequently, Cl.sup..crclbar. concentration.
Such low molecular weight materials have also been found to interfere with other ion-sensitive electrodes, such as silver/silver bromide electrodes for the measurement of bromide ions where chloride and other ions interfere. To solve this problem, layers or films of polymeric materials, such as cellulose esters, silicone rubber and methyl methacrylate have been provided overlying the electrode. For example, U.S. Pat. No. 3,979,274 describes an enzyme electrode overlaid with a thin layer of one of the foregoing polymeric materials to remove interferences from low molecular weight interferents. U.S. Pat. No. 3,591,482 describes a Ag/AgCl chloride ion-selective electrode bearing a thin layer of methyl methacrylate which serves to protect the electrode from physical damage and corrosion while permitting passage of Cl.sup..crclbar.. U.S. Pat. No. 3,694,163 describes an enzyme electrode comprising a membrane of a phase inverted cellulose acetate material containing a reagent and an amide swelling agent. The swelling agent is said to, among other things, control response time to steady state.
Although the foregoing solutions to the interference problem described above are apparently satisfactory for some purposes, I have found that they are not entirely satisfactory when a Ag/Ag halide electrode coated with such overlayer which has been stored for some time is used to assay aqueous solutions, especially body fluids and most specifically blood serum, for halide ion content. Furthermore, many of the foregoing proposed solutions to the interference problem involve further problems. For example, testing has indicated that methyl methacrylate overcoats as thin as 7.mu. for chloride sensitive electrodes provide totally inoperative or radically unstable electrodes. Furthermore, even with cellulose acetate overcoats, the range of useful materials is relatively narrow, i.e., cellulose acetates having an acetyl content below about 36.5 percent tend to dissolve in aqueous solutions, while those having an acetyl content above about 40 percent have too much hydrophobicity to provide desirable relatively short response times.
In copending U.S. application Ser. No. 956,527 entitled "Halide Ion-selective Devices" by Kim, Battaglia and Secord, filed of even data with this application, cellulose acetate overcoats having certain hydrophobic qualities are disclosed as being useful in preventing interference from many low molecular weight materials in halide ion concentration evaluations.
It was discovered, however, that upon aging some change occurs in the overlayer which results in a substantial change in the response time of the electrode (i.e., the time required by the electrode to reach a steady, readable state).
Accordingly, it would be desirable to provide a halide ion-sensitive electrode which possesses the advantages of the electrodes described above and which maintains a rapid response time after aging.